Electrophotographic denitrified glass binder plate

ABSTRACT

A xerographic plate comprising a two-phase layer one component of which is a photoconductive metal oxide which has been recrystallized in a glassy binder. The two-phase photoconductive plate is prepared by mixing a major proportion of the photoconductive metal oxide and minor proportions of other glass formers, fusing the mixture and cooling it so as to form a single glass phase, and then heat treating the single phase glass so as to precipitate the metal oxide as finely divided, uniformly dispersed particles.

Unite States Patent Wood et a1. 45 Apr. 11, 1972 54] ELECTROPHOTOGRAPHIC 2,889,952 6/1959 Claypoole "220/21 DENITRIFIED GLASS BINDER PLATE 3,486,871 12/1971 Martin ..65/33 3,149,016 9/1964 Tun ..16l/1 [72] ll'lVEl'lIOISI Charles WOOd, Sycamore, John C. 3 c i schmmiue" Penfieldi 3 507 646 4/1970 Wood ..96/1 State mega, 2,951,167 8/1960 Kegg et a1. ...313/85 3] A igneeI Xerox Corporation, Stamford, Conn. 3,291,586 12/1966 Chapman et a1 ..65/33 [22] Filed: y 20,1970 3,238,151 3/1966 Kim ..252/518 [21 1 Appl. No.: 39,691 Primary ExaminerCharles E. Van Horn Assistant Examiner-John C. Cooper, 111 Relaled Apphcmlo" Dam Attorneylamcs J. Ralabate, Donald F. Daley and Owen D. [63] Continuation-impart of Ser. No. 539,097, Mar. 31, Manama 1966, abandoned.

57] ABSTRACT [52] U.S. C1. ..96/l.5, 252/501, 106/39 DU, A Xerographic plate comprising a two phase layer one 51 I t Cl 1 106/53 117/129 96/1'8 ponent of which is a photoconductive metal oxide which has d been recrystallized in a glassy binder. The two-phase le 0 can 1 5 i photoconductive plate is prepared by mixing a major proportion of the photoconductive metal oxide and minor proportions of other glass formers, fusing the mixture and cooling it [56] References cued so as to form a single glass phase, and then heat treating the UNITED ST PATENTS single phase glass so as to precipitate the metal oxide as finely divided, uniformly dispersed particles. 2,920,971 l/l960 Stookey ..l06/39 Cianchi ..317/258 6 Claims, 1 Drawing Figure MOLE "/0 PATENTEDAPR 11 1972 3,655,376

YAYAYA MOLE Y NVVVWW 45 MOLE slo INVENTORS CHARLES wo JOHN oscno MILLER BY RUSTUM In;

ATTOI? Y ELECTROPHOTOGRAPHIC DENITRIFIED GLASS BINDER PLATE BACKGROUND OF THE INVENTION This application is a continuation-in-part of Ser. No. 539,097, now abandoned, filed on Mar. 31, 1966.

This invention relates in general to electrophotographic plates, and more specifically to electrophotographic plates having a photoconductive insulating layer comprising a twophase glass-ceramic containing a major proportion of a photoconductive metal oxide crystallized in situ in an amorphous glass binder.

It is known that images may be formed and developed on the surface of certain photoconductive insulating materials by electrostatic meansv The basic electrophotographic process, as taught by Carlson in U.S. Pat. No. 2,297,391, involves uniformly charging a photoconductive insulating layer and then exposing the layer of light-and-shadow image which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light-and-shadow image. Alternatively, a latent electrostatic image may be formed on the layer by charging said layer in image configuration. This image is rendered visible by depositing on the imaged layer a finely divided electroscopic developing material. The powder developing material will normally be attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. Where the base sheet is relatively inexpensive, such as paper, the powder image may be fixed directly to the plate, as by heat or solvent fusing. Alternatively, the powder image may be transferred to a sheet of receiving material such as paper and fixed thereon. The above general process is also described in U.S. Pat. Nos. 2,357,809; 2,891,011, and 3,079,342.

The photoconductive insulating layer to be useful in electrophotography must be capable of holding an electrostatic charge in the dark and dissipating the charge to a conductive substrate when exposed to light. That various photoconductive insulating materials may be used in making electrophotographic plates is known. Suitable photoconductive insulating materials such as anthracene, sulfur, selenium or mixtures thereof have been disclosed by Carlson in U.S. Pat. No. 2,297,691. These materials generally have sensitivity limited to the blue or near ultra-violet range, and all but selenium have a further limitation of being only slightly light sensitive. For this reason, selenium has been the most commercially acceptable material for use in electrophotographic plates. Vitreous selenium, while desirable in most aspects, suffers from serious limitations in that its spectral response is somewhat limited to the ultra-violet, blue and green regions of the spectrum and the preparation of vitreous selenium plates requires costly and complex procedures such as vacuum evaporation. Also, vitreous selenium layers are only metastable in that they are readily recrystallized to inoperative crystalline form at temperatures only slightly in excess of those prevailing in conventional electrophotographic copying machines. Further, selenium plates require the use of a separate conductive substrate layer, preferably with an additional barrier layer deposited thereon before deposition of the selenium photoconductive layer. Because of these economic and commercial considerations, there have been many recent efforts towards developing photoconductive insulating materials other than selenium for use in electrophotographic plates.

It has been proposed that various two-component materials be used in photoconductive insulating layers used in electrophotographic plates. These consist of a photoconductive material in particulate form, dispersed in an insulating binder. Where the particles consist of a photoconductive material comprising an inorganic crystalline compound containing a metallic ion, satisfactory photographic speed and spectral response for use in electrophotographic plates are obtained.

However, these plates, even when dye-sensitized, generally have sensitivities much lower than selenium. These plates are generally considered to be non-reusable since it is necessary to use such high percentages of photoconductive pigment in order to obtain adequate sensitivity that is difficult to obtain smooth surfaces which lend themselves to efficient toner transfer and subsequent cleaning prior to reuse. An additional drawback in the use of inorganic pigment-binder type plates is that they can be charged only by negative and not by positive corona discharge. This property makes them commercially undesirable since negative corona discharge generates much more ozone and is generally more difficult to control.

It has further been demonstrated that organic photoconductive dyes in a wide variety of polycyclic compounds may be used together with suitable resin materials to form photoconductive insulating layers used in binder-type plates. These plates generally lack sensitive levels necessary for use in conventional electrophotographic copying devices. In addition, these plates lack abrasion resistance and stability of operation, particularly at elevated temperatures.

In another type plate inherently photoconductive polymers are used, frequently in combination with sensitizing dyes or Lewis acids, to form photoconductive insulating layers. These polymeric organic photoconductive plates generally have the inherent disadvantages of high cost of manufacture, brittleness, poor adhesion to supporting substances. A number of these photoconductive insulating layers have thermal distortion properties which make them undesirable in an automatic electrophotographic apparatus which often include the powerful lamps and thermal fusing devices which tend to heat the electrophotographic plate.

There has been recently developed a pigment-binder type electrophotographic plate in which the major proportion is a non-photoconductive glass binder. Inorganic photoconductive pigment particles are mixed with glass particles, the glass is fused and the two-phase mixture is coated onto a conductive substrate forming an electrophotographic plate. Such plates are described in detail by Corrsin in U.S. Pat. No. 3,151,982.

These plates have generally excellent physical characteristics in that they have especially smooth, tough surfaces adapted to easy cleaning and are unusually abrasion resistant. However, these plates have several of the disadvantages of the binder-type plates discussed above. In order to produce a smooth surface plate, no more than about 40 percent by weight of the plate consists of photoconductive particulate material. While it would be desirable to include a larger percentage of photoconductive particles for increased photosensitivity, such plates would have very rough, substantially nonreusable surfaces. Since the plates are ordinarily made by mixing photoconductive particles with glass particles and then sintering the glass, it is often difficult to obtain a uniform dispersion of the photoconductive particles throughout the glass binder. Also, the particle size of the photoconductive particles is often non-uniform and the particles often cannot be obtained in as small a size as would be desirable. Plates having a non-uniform dispersion of photoconductive particles or variations in particle size show non-uniform photosensitive response and are not capable of producing optimum images.

Thus, there is a continuing need for improved photoconductive insulating materials from which stable, highly sensitive abrasion resistant and reusable electrophotographic plates can be made.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide electrophotographic plates overcoming the above-noted deficiencies.

Another object of this invention is to provide a binder-type electrophotographic plate having improved uniformity of particle size and particle dispersion in the binder.

Another object of this invention is to provide an electrophotographic plate having improved physical and electrical characteristics.

Another object of this invention is to provide an electrophotographic plate having a smooth, tough abrasion resistant surface.

Still another object of this invention is to provide an electrophotographic plate having a wide range of useful physical properties.

Still another object of this invention is to provide electrophotographic plates suitable for use in both single use and reusable systems.

Still another object of this invention is to provide an electrophotographic imaging process utilizing an electrophotographic plate having high sensitivity, uniform electrical characteristics and a tough, smooth, abrasion-resistant surface.

It is still another object of this invention to provide a process for preparing an electrophotographic plate having a wide range of desirable physical properties.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention, fundamentally, by providing an electrophotographic plate comprising a two-phase layer, the major component of which is a photoconductive metal oxide which has been recrystallized either as a simple or complex oxide in a glassy matrix, and a method of electrophotographic imaging using said plate. This electrophotographic plate is prepared by a devitrification process which includes basically mixing a major proportion of the photoconductive metal oxide and minor proportions of other glass formers, fusing the mixture and cooling it so as to form a single phase glass, then heat treating the glass so as to precipitate the metal oxide as finely divided, uniformly dispersed particles. Such polycrystalline solids prepared by the controlled crystallization of glasses are referred to in the art as glass-ceramics. Glass-ceramics are distinguished from glasses by the presence of substantial amounts of crystals since true glasses are entirely amorphous. Glass-ceramics are described in detail by P. W. McMillan in the book Glass-Ceramics," Academic Press, New York, 1964. Crystallization is accomplished by subjecting the glass to a carefully regulated heat treatment schedule which results in the phase separation and growth of crystal phases within the glass. These glass-ceramics have, in general, much higher mechanical strength than the original glass.

More specifically it has been unexpectedly found that when a glass forming composition having at least 50 mole percent photoconductive metallic oxide is devitrified the photocon ductive metallic oxide recrystallizes as a simple or complex oxide in a glass matrix. This devitrified glass composition has excellent electrophotographic properties which can be partially explained by the chemical equilibria of the reprecipitated oxide with the glassy matrix and the photoconductive metallic oxide proximity caused by the homogeneous nucleation of the oxide, and its complexes, throughout the glass composition. In addition photoconductive insulating layers of the instant composition prepared by the aforementioned recrystallization process are characterized by an unusually uniform distribution of photoconductive crystals of especially small size throughout the glassy binder. This system is capable of including an especially high proportion of photoconductive materials without producing a rough surface. The major component of the photoconductive insulating component of the photoconductive insulating layer consists of one or more photoconductive metal oxides, with the remainder of the layer comprising any glass forming materials in which the metal oxide is highly soluble.

While the prior art teaches compositions of photoconductive metallic oxides in glass binders there is no appreciation of the present devitrified composition with its inherently unique distribution of photoconductive metallic oxides nor of its use in electrophotography. For example, the US. patent to Corr sin, referred to above, discloses the preparation of a photoconductive glass binder photoconductive metal oxides with a glass enamel and fixing composition by mixing the composition to fuse the enamel to a conductive backing which results in a uniform layer of the photoconductive particles embedded in the glass binder. The devitrified composition of the instant invention however is intrinsically distinguishable from the Corrsin composition in that because of the devitrification process the recrystallized photoconductive oxide and their complexes are chemically fixed in the structural network whereas in Corrsin the photoconductive oxides are physically fixed by, i.e., embedded in the glass binder.

The plates produced by this recrystallization process are characterized by an unusually uniform dispersion of photoconductive crystals of especially small size throughout the glassy binder. This system is capable of including an especially high proportion of photoconductive materials without producing a rough surface. The major component of the photoconductive insulating layer consists of one or more photoconductive metal oxides, with the remainder of the layer comprising any glass forming materials in which the metal oxide(s) is highly soluble.

DESCRIPTION OF THE DRAWINGS Referring now to the FIGURE, there is shown a conventional ternary composition diagram for a three-component glass comprising lead oxide, silicon dioxide, and aluminum oxide. This diagram is typical of those which may be drawn for any of the other compositions of this invention. The diagram is merely exemplary of the invention in describing a specific em bodiment thereof. The percentages along each axis are in mole percent. The hatched area delineates the approximate compositions which are capable of producing, when heat treated, precipitated lead oxide particles in a glassy binder.

Compositions made up of proportions indicated by the dots at 1-3 on the diagram prepared as in Example llll were amorphous in nature and could not be recrystallized by any of the usual heat treatments. The compositions having ingredients proportioned as indicated by dots 4-17, prepared as in Examples IV-XVIl could not be retained as glasses, that is, they entirely crystallized immediately from the melt upon initial cooling. Compositions having ingredient proportions indicated by the dots at 18-23 prepared as in Examples XVIILXXIII were heat treated to yield lead oxide particles in a glassy binder. As can be seen from the diagram, that area encompassed by points 182319-20-2122 which corresponds to from about 65 to about 75 mole percent lead oxide is preferred so as to produce a glass-ceramic having lead oxide recrystallized in an amorphous glass matrix. With other glass matrix materials, the preferred percentage of lead oxide may vary slightly. As indicated by this exemplary figure, the major portion of the composition should be the photoconductive metal oxide in order that a uniform glassy melt can be obtained, from which the photoconductor can later be recrystallized. The large proportion of photoconductor also results in a highly photosensitive plate.

Any suitable photoconductive metal oxide which is soluble in glass forming materials and can be recrystallized in said materials may be used. Typical photoconductive metal oxides include Pb0, Zn0, Ba0, Ti0 Cd0, Bi 0 Ga 0, ln 0 Sn0 Sb 0 Te0 Cu 0, As 0 and mixtures thereof. Lead oxide has been found to give especially good results and, therefore, is the preferred photoconductive metal oxide. Plates including lead oxide are unusually durable and are highly photosensitive. While these plates are sensitive to visible light and are useful in conventional electrophotography, they are also sensitive to X-rays and are especially useful in electroradiography. Exemplary systems of electroradiography for which these plates are suitable include those described by Schaffert in U.S. Pat. No. 2,666,144.

Any suitable glass forming material in which the desired photoconductive metal oxide is soluble may be used. Typical glass forming compositions include A5 0 Si0 B 0 P 0 Sb 0 GeO V 0 and mixtures thereof.

The single phase glassy layer may include any other suitable material, where desired. For example, spectral sensitizing materials or physical property modifying materials may be included. Typical materials which may be included to modify the electrical or other physical properties of the plate include A1 0 MgO, Li 0, Na O, K 0, CaO, SrO and mixtures thereof.

The two-phase glass-ceramic photoconductive insulating layer of this invention may be deposited on any suitable supporting substrate or may be cast as a self-supporting sheet. The plate may be overcoated with any suitable material, if desired. The photoconductive insulating layer may be used in the formation of multi-layer sandwich configurations adjacent a dielectric layer, similar to those shown by Golovin et al., in the publication titled A New Electric Photographic Process Effected by Means of Combined Electret Layers, Doklady. Akad. Nauk SSSR, Vol. 129, No. 5, 1008-1011, November- December, 1959. Where the photoconductive insulating layer is coated on a substrate, a wide variety of materials may be used, for example, metal surfaces such as aluminum, brass, stainless steel, copper, nickel, zinc etc.; conductively coated glass such as tin or indium coated glass, aluminum coated glass, etc.; under certain conditions such as at higher temperatures, common plate glass has a sufficiently low resistivity to act as a ground plane. In general, to act as a ground plane as described herein, a backing material may have a surprisingly high resistivity, such as 40 ohm-cm. The material must, where the layer is formed directly on the substrate, be capable of withstanding the temperatures required for fusing and heat treatment of the glass photoconductive insulating layer.

The following examples further specifically define and describe methods of making the two-phase glass-ceramic photoconductive layers of the present invention. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I About 85 (60 mole percent) powdered PbO is mixed under acetone with about parts (40 mole percent) powdered Si0 in an alumina morter. The mixture is dried at about 100 C for about 30 minutes in a platinum crucible. The crucible is then placed in a flask at about 900 C and held at about that temperature for about 15 minutes. The melted glass is then poured into a beaker of distilled water, dried at a temperature of about 100 C and remelted. The melt is then poured onto a stainless steel sheet preheated to about 200 C. This sample is entirely amorphous. Attempts to cause recrystallization of the lead oxide portion by heat treating at temperatures ranging from about 350 C to about 500 C for periods ranging from about 8 hours to about hours fail to produce any recrystallization. This composition is shown at l in the FIGURE.

EXAMPLE II The experiment of Example I is repeated with a mixture consisting of about 87 parts PbO (65 mole percent) and about 18 parts Si0 (35 mole percent). Again, the product is uniformly amorphous and attempts to recrystallize the Pb0 phase fail. This composition is shown at 2 in the FIGURE.

EXAMPLE III The experiment of Example I is repeated with a mixture consisting of about 87 parts Pb0 (65 mole percent), about 10 parts Si0 mole percent) and about 3 parts A1 0 (10 mole percent). Again, the sample produced is entirely amorphous and attempts to recrystallize the Phi) phase are not successful. This composition is shown at 3 in the FIGURE.

EXAMPLE IV About 84 parts P190 (65 mole percent) is mixed under acetone with about 5 parts SiO (15 mole percent) and about 1 1 parts A1 0 (20 mole percent) in an alumina mortar. The mixture is dried at about 110 C for about minutes in a platinum crucible. The crucible is then placed in a furnace at about 900 C and held at about that temperature for about 15 minutes. The molten glass is then poured into a beaker of distilled water, and dried at about 100 C. The material appears to be entirely crystallized with no amorphous phase present. The material is then remelted and poured onto a stainless steel sheet heated to about 200 C. Again, the entire mixture appears to be crystallized with no amorphous phase present. This composition is shown at 4 in the FIGURE.

EXAMPLE V The experiment of Example IV is repeated with a mixture consisting of about parts Pb0 (66 mole percent), about 7 parts Si0 (18 mole percent) and about 8 parts A1 0 (16 mole percent). The glass entirely crystallizes upon solidification from the melt. This composition is shown as 5 in the FIGURE.

EXAMPLE VI The experiment of Example IV is repeated with a mixture consisting of about 84 parts Pb0 (67 mole percent), about 5 parts Si0 (13 mole percent) and about 11 parts A1 0 (20 mole percent). This material entirely crystallizes upon solidification from the melt. This composition is shown at 6 in the FIGURE.

EXAMPLE VII The experiment of Example IV is repeated with a mixture consisting of about 84 parts PM (67 mole percent), about 5 parts Si0 (13 mole percent) and about 11 parts A1 0 (20 mole percent). This material entirely crystallizes upon solidification from the melt. This composition is shown at 7 in the FIGURE.

EXAMPLE VIII The experiment of Example IV is repeated with a mixture consisting of about parts PbO (70 mole percent) and about 10 parts Si0 (30 mole percent). Again, the entire mass crystallizes immediately upon solidification. This composition is shown at 8 in the FIGURE.

EXAMPLE IX The experiment of Example IV is repeated with a mixture consisting of about 89 parts Pb0 (70 mole percent), about 6 parts Si0 (18 mole percent) and about 5 parts A1 0 12 mole percent). Again, the entire mass crystallizes immediately upon solidification. This composition is shown at 9 in the FIGURE.

EXAMPLE X The experiment of Example IV is repeated with a mixture consisting of about 87 parts Pb0 (70 mole percent), about 5 parts Si0 (15 mole percent) and about 8 parts A1 0 (5 mole percent). Again, the material entirely crystallizes upon solidification from the melt. This composition is shown at 10 in the FIGURE.

EXAMPLE XI The experiment of Example IV is repeated with a mixture of about 90 parts PbO (72 mole percent), about 7 parts Si0 (22 mole percent) and about 3 parts A1 0 (6 mole percent). Again, the material crystallized immediately upon solidification from the melt. This composition is shown at 11 in the FIGURE.

EXAMPLE XII The experiment of Example IV is repeated with a mixture consisting of about 90 parts PbO (73 mole percent), about 7 parts SiO (20 mole percent) and about 3 parts A1 0 (7 mole percent). Again, the material crystallized immediately upon solidification from the melt. This composition is shown at 12 in the FIGURE.

EXAMPLE'XIII The experiment of Example IV is repeated with a composition consisting of about 89 parts PbO (72 mole percent), about parts Sit] (17 mole percent) and about 6 parts A1 0 (1 I mole percent). The material crystallized immediately upon solidification. This composition is shown at 13 in the FIGURE.

EXAMPLE XIV The experiment of Example IV is repeated with a material consisting of about 87 parts Pb0 (72 mole percent), about 3 parts Si0 mole percent) and about 10 parts A1 0 (18 mole percent). Again, the material crystallized upon solidification. This composition is shown in the FIGURE at 14.

EXAMPLE XV The experiment of Example IV is repeated with a composition consisting of about 89 parts Pb() (74 mole percent), about 4 parts Si0 (14 mole percent) and about 7 parts A1 0 (12 mole percent). This composition crystallized immediately upon solidification from the melt. This composition is shown in the FIGURE at 15.

EXAMPLE XVI The experiment of Example IV is repeated with a material consisting of about 9l parts Pb!) (75 mole percent), about 7 parts Si0 (20 mole percent) and about 2 parts A1 0 (5 mole percent). Again, the material crystallized immediately upon solidification. This composition is shown at 16 in the FIGURE.

EXAMPLE XVII The experiment of Example XV is repeated with a material consisting of about 92 parts Pb0 (80 mole percent), 4 parts Si0 (13 mole percent) and about 4 parts A1 0 (7 mole percent). This material also crystallized immediately upon solidification. This composition is shown at 17 in the FIGURE.

EXAMPLE XVIII About 88 parts Pb0 (70 mole percent) is mixed under acetone with about 9 parts Si0 (25 mole percent) and about 3 parts A1 0; (5 mole percent). The mixture is dried at about 120 C for about 20 minutes in a platinum crucible. The crucible is then placed in a furnace at about 900 C and held at about that temperature for about minutes. The molten glass is then poured into a beaker of distilled water, and dried at about 100 C. The material is then remelted and then poured onto a stainless steel sheet preheated to about 200 C and gradually cooled to room temperature. The sample is entirely amorphous. The sample is then heated to a temperature of about 450 C for about 5 hours, and then returned to room temperature. Examination shows a uniformly dispersed crystalline phase throughout the glassy binder. This crystalline phase appears to be entirely lead oxide. This composition is shown at 18in the FIGURE.

EXAMPLE XIX The experiment of Example XVIII is repeated with a mixture consisting of about 87 parts Pbt) (70 mole percent), about 7 parts Si0 mole percent) and about 6 parts A1 0 (10 mole percent). This material is entirely glassy when first produced and upon heat treatment the lead oxide phase recrystallizes out as a uniform dispersion of fine crystals. This composition is shown at 19 in the FIGURE.

EXAMPLE XX The experiment of Example XVIII is repeated with a mixture consisting of about 86 parts PbO (67 mole percent), about 7 parts SiO (20 mole percent) and about 7 parts A1 0 (13 mole percent). When first solidified from the melt, this material is entirely glassy. Upon heat treatment, the lead oxide precipitates as a uniform dispersion of small crystals throughout the glassy phase. This composition is shown at 20 in the FIGURE.

EXAMPLE XXI The experiment of Example XVIII is again repeated with a mixture of about 87 parts Pb0 (67 mole percent), about 5 parts A1 0 (9 mole percent), and about 8 parts Si0 (24 mole percent). This material is entirely glassy when first produced and upon heat treatment the lead oxide phase recrystallizes out as a uniform dispersion of fine crystals. This composition is shown at 21 in the FIGURE.

EXAMPLE XXII The experiment of Example XVIII is repeated with a mixture consisting of about 87 parts Pb0 (68 mole percent), about 3 parts A1 0 (5 mole percent), and about 10 parts SiO (27 mole percent). As in Examples XVIII and XXI when first solidified from the melt, the material is entirely glassy. Upon heat treatment, the lead oxide precipitates as a uniform dispersion of small crystals throughout the glassy phase. This composition is shown at 22 in the FIGURE.

EXAMPLE XXIII The experiment of Example XVIII is again repeated with a mixture of about 89 parts PbO (71 mole percent), about 4 parts A1 0 (7 mole percent), and about 7 parts Si0 (22 mole percent). Again as in Examples XVIII through XXII the devitrification process resulted in the reprecipitation of the lead oxide as a uniform dispersion of small crystals throughout the glassy binder. This composition is shown at 23 in the FIGURE.

EXAMPLE XXIV About 87 parts (69 mole percent) powdered PhD is mixed under acetone with about 6 parts 10 mole percent) powdered A1 0 and about 7 parts (21 mole percent) Si0 in an alumina morter. The mixture is dried at about C for about 30 minutes in a platinum crucible. The crucible is then placed in a furnace at about 850 C and held at about that temperature for about 15 minutes. The fused glass is then poured into a beaker of distilled water, dried at 1 10 C and remelted. The melt is then poured onto a stainless steel sheet preheated to about 200 C. The sample is divided into six portions, each of which is heat treated as below described.

Sample A is heated to a temperature of 370 C for about 16 hours. Upon cooling to room temperature and examination by polariscope and X-ray diffraction techniques, the sample is found to be entirely amorphous, with no precipitated phase.

Sample B is heated to about 500 C for about 10 hours, then cooled to room temperature. Examination shows a crystalline phase in the glass binder. The crystalline phase appears to be a mixture of lead oxide and PbA1 0 Sample C is heated to a temperature of 450 C for about 5 hours, then returned to room temperature. Examination shows a crystalline phase dispersed throughout the glassy binder. This crystalline phase appears to be entirely lead oxide. Sample D is heated to a temperature of about 425 C for about 2 hours, then cooled to room temperature. This glass is found to comprise a small amount of crystallized lead oxide in a glassy binder. Sample E is heated to a temperature of about 425 C for about 3 hours, then cooled to room temperature. This sample is found to contain a higher percentage of crystallized lead oxide dispersed in the glassy binder. Sample F is heated to a temperature of about 425 C for about 4 hours, then cooled to room temperature. Upon examination it is found that substantially all of the lead oxide has crystallized in the glassy binder.

As can be seen from the treatment of the above samples, a crystalline lead oxide phase in a glassy binder may be produced by heating the amorphous original glass to a tem perature of at least 425 C for at least 2 hours. For optimum recrystallization of the lead oxide in the glassy binder, temperature of at least 425 C.

About 83 parts (65 mole percent) powdered PbO is mixed under acetone with about 9 parts (15 mole percent) powdered Get) and about 8 parts (20 mole percent) powdered B in an alumina morter. The mixture is dried at about 100 C for about 30 minutes in a platinum crucible. The crucible is then placed in a furnace at about 900 C and held at about that temperature for about 15 minutes. The fuse glass is then poured into a beaker of distilled water, dried at about 100 C and remelted. The melt is then poured onto a stainless steel sheet preheated to about 200 C. The material is now in a glassy state. The sample is then heated to a temperature of about 500 C for about hours and then slowly returned to room temperature. Examination shows a crystalline phase dispersed uniformly throughout the glassy binder. This crystalline phase appears to be entirely lead oxide.

EXAMPLE XXVI About 84 parts (67 mole percent) powdered PbO is mixed under acetone with about 11 parts (18 mole percent) powdered GeO and about 5 parts (14 mole percent) 13 0 in an alumina mortar. The mixture is dried at about 120 C for about 20 minutes in a platinum crucible. The crucible is then placed in a furnace at about 950 C and held at about that temperature for about 20 minutes. The fused glass is then poured into distilled water, dried at about 100 C and remelted. The melt is then poured onto a stainless steel sheet preheated to about 250 C. The sample is now uniformly glassy. The sample is then heated to a temperature of about 450 C for about 4 hours, then returned to room temperature. Examination shows a crystalline phase uniformly dispersed in the glassy binder. The crystalline phase appears to be entirely lead oxide.

EXAMPLE XXVIl About 86 parts (65 mole percent) powdered Pb0 is mixed under acetone with about 8 parts (20 mole percent) B 0 and about 6 parts Si0 (15 mole percent). The mixture is dried at about 1 10 C for about 20 minutes in a platinum crucible. The crucible is then placed in a furnace at about 850 C and held at about that temperature for about minutes. The fused glass is then poured into a beaker of distilled water, dried at about l00 C and remelted. The melt is then poured onto a stainless steel sheet preheated to about 200 C. The sample produced is uniformly amorphous. This sample is then preheated to a temperature of about 480 C for about 8 hours, then cooled to room temperature. Examination shows a crystalline phase in the glass binder. The crystalline phase appears to be lead oxide.

EXAMPLE XXVlll About 88 parts (67 mole percent) Pb0 is mixed under acetone with about 6 parts 15 mole percent) B 0 and about 6 parts Sit) 18 mole percent). The mixture is dried at about 100 C for about 30 minutes in a platinum crucible. The cmcible is then placed in a furnace at about 850 C and held at about that temperature for about 15 minutes. The fused glass is then poured into distilled water, dried at about 100 C and remelted. The melt is then poured under a stainless steel sheet preheated to about 200 C. The sample at this time is entirely glassy. The sample is then heated to a temperature of about 500 C for about 6 hours, then returned to room temperature. Examination shows a uniform crystalline phase uniformly dispersed throughout the glassy binder. The crystalline phase appears to be entirely lead oxide.

EXAMPLE XXIX A mixture consisting of about 50 mole percent zinc oxide and about 50 mole percent germanium oxide is placed in a sealed tube and heated to about 150 C, then quenched. The entire sample is glassy. The sample is then reheated to about 500 C for about 12 hours. Examination of the sample shows a uniform crystalline phase uniformly dispersed throughout a glassy binder. The crystalline phase appears to be principally zinc oxide.

EXAMPLE XXX A mixture consisting of about 60 mole percent HgO and about 40 mole percent P 0 is placed in a sealed tube and melted at about 600 C. The sample is quenched, producing an amorphous solid. The sample is then reheated to about 800 C and held at that temperature for about 72 hours. After cooling to room temperature a uniform crystalline phase is observed dispersed throughout a glassy matrix. The crystalline phase appears to be principally precipitated H g0.

The two-phase glass-ceramic photoconductive insulating layers prepared as in the above examples, are useful in electrophotographic plates in electrophotographic imaging processes. These materials are also useful in electroradiographic imaging processes.

The following examples further specifically define the present invention with respect to the two-phase glass-ceramic electrophotographic plates and imaging processes using them. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the electrophotographic plates and electrophotographic imaging processes of the present invention.

EXAMPLE XXXl A sheet of the glass-ceramic prepared as in Example XVlll above, having a thickness of about 500 microns is bonded to an aluminum substrate with Silverprint," a mixture of silver powder in an adhesive carrier, available from General Cement Electronics Co. The plate is charged to a negative potential of about 2,000 volts in the dark by corona discharge, as described in U.S. Pat. No. 2,777,957. A conventional black and white transparency is placed about 1 millimeter from the surface of the plate. The plate is illuminated through the transparency by means of a l00-watt Burton lamp, available from the Burton Manufacturing Co. The lamp is held about 1 inch from the plate. The plate is exposed to the image for about 60 seconds. The image is developed by cascading electroscopic marking particles over its surface by the process described by Walkup in U.S. Pat. No. 2,618,551. A powder image is formed on the plate surface corresponding to the transparency image. The powder image is transferred to a sheet of ordinary bond paper by the method described by Schaffert in U.S. Pat. No. 2,576,047. The paper sheet is then heated to the melting point of the electroscopic marking particles and cooled; forming a permanent image of good quality confirming to the original. The plate may then be reused, as by the above process.

EXAMPLE XXXll A sheet of the glass-ceramic prepared as in Example XX, having a thickness of about 600 microns, is bonded to an aluminum substrate with Silverprint. The plate is then charged and exposed as in Example XXXl above. The resulting electrostatic latent image is developed by cascading electroscopic marking particles over the plate. The plate is heated until the particles melt, then cooled to room temperature. A positive image conforming to the original results.

EXAMPLE XXXlII A sheet of the glass-ceramic prepared as in Example XXIX having a thickness of about 400 microns is electrostatically charged to a potential of about 2,000 volts by means of two corona units simultaneously charging both sides of the sheet, as described by Gundlach in U.S. Pat. No. 2,885,556. The charged plate is then exposed to a light-and-shadow image by means of a conventional transparency. The plate is illuminated by a lOO-watt Burton lamp, held about 1 inch from the plate, for about 70 seconds. The image is developed, transferred to a paper sheet, and fused as in Example XXXI. The resulting image is of good quality, conforming to the original.

Although specific components and proportions have been described in the above examples of methods of preparing photoconductive two-phase glass-ceramics and of forming electrophotographic images on two-phase glass-ceramic plates, other suitable materials as listed above, may be used with results. In addition, other materials may be added to the glass compositions to synergize, enhance, or otherwise modify their properties. For example, spectral sensitizing agents may be added to the glass compositions to modify the spectral response of the plates.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. The preparation of an electrophotographic plate comprising:

a. providing a mixture of PhD, A1 and Si0 in concentrations corresponding to the area encompassed by points 18-23-19-20-21-22 of the ternary diagram in the drawb. coating said mixture onto a supporting conductive substrate;

c. heating said coated substrate above the melting point of the coating ingredients;

d. cooling said coated substrate whereby the coating comprises a single phase glassy body; and

e. reheating to a temperature at which the lead oxide precipates with a finely divided, uniformly distributed crystal phase.

2. The method of claim 9 wherein the mixture additionally includes a minor proportion of an additive selected from the group consisting essentially of MgO, Li 0, Na,0, K 0, CaO, Sr0 and mixtures thereof.

3. The process of claim 9 wherein the PM is in a concentration of from about 65 to 75 mole percent, and the reheating takes place at a temperature of at least 425 C for at least 2 hours.

4. The plate produced for the process of claim 9.

5. The preparation of an electrophotographic composition comprising:

a. providing a mixture of PM), A1 0 and Si0 in concentrations corresponding to the area encompassed by points 18-23-19-20-21-22 of the ternary diagram in the drawmg;

b. heating said mixture above the melting points of the ingredients;

c. cooling said mixture whereby single phase glassy body is formed; and

d. reheating said body to a temperature of at least 425 C for at least 2 hours wherein the PbG precipitates the finely divided, uniformly distributed crystal phase.

6. The product produced by the process of claim 5.

Twi UNITED STATES PATENT OFFICE CERTIFICATE GP CURREC'HUN Patent No. 3,655,376- Dated April 11, 1972 lnvent Charles Wood et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 2, column 12, line 5, delete "9" and insert l.

Claim 3 column 12, line 9, delete "9" and insert l.

Claim4, column 12, line 13, delete "9" and insert l.

Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCI-ER,JR. ROERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The method of claim 9 wherein the mixture additionally includes a minor proportion of an additive selected from the group consisting essentially of Mg0, Li20, Na20, K20, Ca0, Sr0 and mixtures thereof.
 3. The process of claim 9 wherein the Pb0 is in a concentration of from about 65 to 75 mole percent, and the reheating takes place at a temperature of at least 425* C for at least 2 hours.
 4. The plate produced for the process of claim
 9. 5. The preparation of an electrophotographic composition comprising: a. providing a mixture of Pb0, A1203, and Si02, in concentrations corresponding to the area encompassed by points 18-23-19-20-21-22 of the ternary diagram in the drawing; b. heating said mixture above the melting points of the ingredients; c. cooling said mixture whereby single phase glassy body is formed; and d. reheating said body to a temperature of at least 425* C for at least 2 hours wherein the Pb0 precipitates the finely divided, uniformly distributed crystal phase.
 6. The product produced by the process of claim
 5. 